Data may typically be written onto a storage medium, such as magnetic or magneto-optic tape, in the form of a sequence of data blocks identified by a unique block identifier. A varying number of these data blocks may be grouped together to form a data group or other group that may be used to designate certain areas of the medium (for example “End Of Data”). The data group is typically the smallest entity that can be written to this medium and contains user data.
The operation of adding new data groups to a medium which already has existing data groups written upon it is usually referred to as “appending.” In addition data groups may be written over previously written data with the result that there may be a boundary at one or both ends of the newly written data between previously written and the newly written data. For convenience, in the description that follows such newly written data is also referred to as “appended.”
One method of storing and reading data on tape media is to use helical scan recording. In a helical scan recording/read apparatus (usually referred to as a ‘tape drive’) data is recorded on and read from a tape by a rotating drum carrying one or more electromagnetic heads. The tape is moved by a motor driven capstan along a path extending between two tape reels and partially around the drum. The plane of rotation of the drum is inclined to the plane of the tape so that the electromagnetic head(s) traverse the tape along successive tracks extending across the width of the tape at an angle to its centreline.
To obtain optimum data retrieval during a read operation, the electromagnetic read head(s) of a helical scan tape drive have to “lock” on the data on the tape so as to accurately follow the tracks across the tape. This requires control of the relative motion of the tape and drum by controlling rotation of the drum and/or movement of the tape by the capstan. The control process includes using a track reference value (“Time-Tracking measurement value”), which is representative of the average spacing of the data tracks from the lower edge of the tape.
The conditions during an append operation may differ from those prevailing during a previous write operation resulting in the tracks of the appended data groups being at a different height to the tracks of the previously written data. This is most likely to happen where the appended data is written by a different tape drive, although other factors, such as temperature, wear or changes of settings of the tape drive components or wear of the tape, may also produce a change in cases in which the same tape drive is used.
Often, a tape drive can cope with differences in the height of the tracks before and after the append point. However, the difference can cause the tape drive to lose lock at the append point. If that happens, the tape drive has to back over the append point and perform a Time-Tracking calibration, measuring the height of the data after the append point and sometimes before the append point as well. Sometimes this Time-Tracking calibration and subsequent retries fail, even though the tape drive has a good error performance. This is generally due to the appended data being written at an extreme height difference or because there is some feature of the written data at the append point where the Time-Tracking measurements are performed that causes an inadequate measurement (for example a localised bad error rate).
Even when Time-Tracking calibration is successful, the process of making the measurements slows the operation of the tape drive and leads to increased wear of the tape.